In recent years, large-capacity and long-distance transmission has become desirable in optical transmission systems. Therefore, it is desirable to employ a method for compensating waveform degradation due to nonlinear optical effects such as self-phase modulation (SPM) and cross-phase modulation (XPM).
For example, SPM includes, in a broad sense, intra-channel cross-phase modulation (IXPM) caused by a nonlinear optical effect in a channel. As a method for compensating waveform degradation due to IXPM, for example, a nonlinear compensation method that uses perturbation back-propagation (PBP) is provided. In the PBP scheme, a perturbation analysis is performed on the Manakov equation to calculate a nonlinear distortion, and the calculated nonlinear distortion is reduced. In this manner, the waveform degradation due to SPM such as IXPM may be compensated at a receiving terminal.
As a factor contributing to the waveform degradation due to XPM, for example, inter-polarization crosstalk caused by a nonlinear optical effect between channels is conceivable. As a method for compensating the waveform degradation due to XPM, a nonlinear polarization crosstalk canceller (NPCC) scheme is provided. In the NPCC scheme, an inter-polarization crosstalk coefficient is calculated based on a first polarization signal and a second polarization signal that are received at a receiving terminal. The second polarization signal is a polarization signal orthogonal to the first polarization signal. In the NPCC scheme, the waveform degradation due to, for example, inter-polarization crosstalk caused by XPM may be compensated at the receiving terminal based on the calculated inter-polarization crosstalk coefficient.
Japanese Laid-open Patent Publication No. 2012-186806, Japanese Laid-open Patent Publication No. 2012-186807, Japanese National Publication of International Patent Application No. 2013-509747, and Japanese Laid-open Patent Publication No. 2012-090262 are examples of the related art.
When the compensation method that uses, for example, the PBP scheme or the NPCC scheme is employed, however, operation parameters are used and the maximum compensation effect is not attainable unless optimum values are set for the operation parameters. For example, in the PBP scheme, it is desirable that the number of filter taps, a gamma coefficient, and the like of the parameters to be used in the PBP scheme be set to optimum values. In the NPCC scheme, it is desirable that an averaging length of the parameters to be used in the NPCC be set to an optimum value. Regarding the averaging length, inputs of a set averaging length (averaging length of N (integer) corresponds to N samples) are averaged and output as processing of a digital filter.
When the PBP scheme or the NPCC scheme is employed, it is desirable to provide a receiving apparatus capable of setting optimum operation parameters in a short period of time while reducing the circuit scale.